The present invention relates to an optical compensatory sheet comprising a transparent substrate and an optically anisotropic layer comprising discotic liquid crystal molecules. The invention also relates to a liquid crystal display of an STN mode equipped with the optical compensatory sheet.
A liquid crystal display of an STN mode comprises a liquid crystal cell of an STN (Super Twisted Nematic) mode, two polarizing elements placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) placed between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The. rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. The rod-like liquid crystal molecules are twisted by using a chiral agent. A twist angle of the molecules is in the range of 90xc2x0 to 360xc2x0.
Without an optical compensatory sheet, a liquid crystal display of an STN mode gives an image colored blue or yellow because of birefringence of rod-like liquid crystal molecules. The blue or yellow color is inconvenient for not only a black and white image but also a color image. An optical compensatory sheet has a function of removing color from an image to display a bright and clear image. The optical compensatory sheet also has a function of enlarging a viewing angle of a liquid crystal cell. As the optical compensatory sheet, a stretched birefringent film has been usually used. Japanese Patent Provisional Publication Nos. 7(1995)-104284 and 7(1995)-13021 disclose an optical compensatory sheet made of a stretched birefringent film for a liquid crystal display of an STN mode.
In place of the stretched birefringent film, an optical compensatory sheet comprising an optically anisotropic layer on a transparent substrate has been proposed. The optically anisotropic layer is formed by aligning discotic liquid crystal molecules and fixing the aligned molecules. The discotic liquid crystal molecules usually have large birefringence and various alignment forms. Accordingly, an optical compensatory sheet obtained by using the discotic liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic liquid crystal molecule is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. However, the disclosed optical compensatory sheet is designed to be used in a liquid crystal display of a TN (Twisted Nematic) mode.
The optical compensatory sheet containing the discotic liquid crystal molecules should be redesigned to be used in a liquid crystal display of an STN mode. In a liquid crystal cell of the STN mode, rod-like liquid crystal molecules are aligned according to a super twisted birefringent mode at a twist angle of larger than 90xc2x0. The liquid crystal display of the STN mode can give a clear image with a large capacity according to a time-sharing addressing method, even if the display has a simple matrix electrode structure (having no active matrix such as a thin film transistor or a diode).
The discotic liquid crystal molecules should be essentially vertically aligned (homogeneously aligned) to optically compensate the liquid crystal cell of the STN mode. The discotic liquid crystal molecules are preferably further twisted. Japanese Patent Provisional Publication No. 9(1997)-26572 discloses an optical compensatory sheet in which discotic liquid crystal molecules are twisted. The drawings of Japanese Patent Provisional Publication No. 9(1997)-26572 further illustrate that the discotic liquid crystal molecules are essentially vertically aligned.
It is technically difficult to align discotic liquid crystal molecules uniformly (i.e., to orient the molecules in mono domain alignment) from the interface facing an orientation layer to the opposite interface facing the air, according to the disclosures of Japanese Patent Provisional Publication No. 9(1997)-26572. If the discotic liquid crystal molecules are not uniformly aligned (oriented in dual domain alignment), disclination of the molecules scatters light to decrease a contrast ratio of a displayed image.
Rod-like liquid crystal molecules used in a liquid crystal cell have been investigated to align the molecules essentially vertically (homeotropic alignment). For example, a liquid crystal display of a vertical alignment (VA) mode uses an orientation layer having a function of essentially vertically aligning rod-like liquid crystal molecules. In the liquid crystal display of the VA mode, rodlike liquid crystal molecules are essentially vertically aligned while voltage is not applied to the cell, and are essentially horizontally aligned while voltage is applied to the cell. Various orientation layers have been proposed to align rod-like liquid crystal molecules.
However, the orientation layer for rod-like liquid crystal molecules cannot sufficiently orient the discotic liquid crystal molecules to form uniform mono domain alignment from an interface facing the orientation layer to the opposite interface facing the air.
An object of the present invention is to provide an optical compensatory sheet particularly suitable for a liquid crystal display of an STN mode.
Another object of the invention is to provide an optical compensatory sheet in which discotic liquid crystal molecules are oriented essentially vertically (homogeneously) and uniformly in mono domain alignment.
A further object of the invention is to provide a liquid crystal display of an STN mode that can display a clear image of a high contrast, in which blue or yellow color caused by birefringence is reduced.
The present invention provides an optical compensatory sheet which comprises a transparent substrate and an optically anisotropic layer comprising discotic liquid crystal molecules, wherein the discotic liquid crystal molecules are oriented in essentially mono domain alignment at an average inclined angle of 50xc2x0 to 90xc2x0, said average inclined angle being an average of angles between discotic planes of said discotic liquid crystal molecules and a surface of said transparent substrate.
The invention also provides a liquid crystal display comprising a liquid crystal cell of an STN mode, two polarizing elements placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets placed between the liquid crystal cell and the polarizing element; wherein the optical compensatory sheet comprises a transparent substrate and an optically anisotropic layer comprising discotic liquid crystal molecules in this order from the polarizing element side, said discotic liquid crystal molecules being oriented in essentially mono domain alignment at an average inclined angle of 50xc2x0 to 90xc2x0 and further being twisted at a twist angle of 90xc2x0 to 360xc2x0, and said average inclined angle being an average of angles between discotic planes of said discotic liquid crystal molecules and a surface of said transparent substrate.
In the present specification, the term xe2x80x9caverage inclined anglexe2x80x9d means an average of angles between discotic planes of discotic liquid crystal molecules and a surface of a transparent substrate (or a surface of an orientation layer). The present specification refers to alignment of liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0 as essentially vertical alignment of the molecules.
It can be easily confirmed by observation with a polarizing microscope whether discotic liquid crystal molecules are oriented in mono domain alignment or not (in dual domain alignment).
The applicants have succeeded in aligning discotic liquid crystal molecules essentially vertically (homogeneously) and uniformly in mono domain alignment.
For example, the discotic liquid crystal molecules can be thus aligned by using an appropriate amount of a specific additive (cellulose esters or alignment promoters) and by controlling the temperature for alignment. Without either using the additives or controlling the aligning temperature, the liquid crystal molecules are aligned in dual domain alignment.
An optical compensatory sheet suitable for a liquid crystal display of an STN mode is now obtained by using the discotic liquid crystal molecules in essentially vertical, uniform and stable alignment. Accordingly, blue or yellow color can be reduced in a liquid crystal display of an STN mode. Therefore, the liquid crystal display of an STN mode can give a clear image with a high contrast by using the optical compensatory sheet in which the discotic liquid crystal molecules are essentially vertically oriented in essentially mono domain alignment (and are preferably twisted).